The invention concerns a method for mounting semiconductor chips and an apparatus suitable for carrying out the method.
Soldering methods of this kind are typicallyxe2x80x94however not exclusivelyxe2x80x94used for the mounting of semiconductor chips on a metallic carrier, a so-called leadframe. Above all, power semiconductors are bonded to the substrate (normally copper) as a rule with soft solder in order to guarantee effective dissipation of the heat loss created in operation by the semiconductor chip by means of the solder connection.
An apparatus for the application of liquid solder to a substrate and for the subsequent mounting of a semiconductor chip onto the liquid solder portion is known from the U.S. Pat. No. 5,878,939. Under the name of xe2x80x9cDie Bonder 2007 SSIxe2x80x9d the applicant offers an apparatus of this kind whereby the semiconductor chip is placed over the solder portion on the substrate by means of a process known as xe2x80x9cOvertravelxe2x80x9d. With this process, the bondhead is lowered so far that, on impact with the solder portion, the gripper holding the semiconductor chip is deflected towards the bondhead. In this way, variations in the thickness of the semiconductor chip, which typically amount to up to 40 xcexcm, and variations in the height of the surface of the substrate can be overcome without problem.
The liquid soft solder has a very low viscosity and, on impact of the semiconductor chip, behaves like water, ie, it flows easily and almost instantly from underneath the semiconductor chip and spreads outside the semiconductor chip. Examinations with a high-speed camera have shown that in doing so a gap remains between the semiconductor chip and the substrate which is in fact filled with solder but the thickness of which only amounts to several micrometres. When the bondhead moves away, ideally a large part of the solder flows back into the gap between the semiconductor chip and the substrate, whereby the gap is again increased, the thickness of the solder layer however is subject to certain deviations even when the semiconductor chip is moved back and forth parallel to the surface of the substrate. Often, the solder does not flow back under the semiconductor chip but settles in beads next to the semiconductor chip. This results in very thin solder layers. In event that the solder flows back only partially or on one side, this results in solder layers with large tilt. The flowing back of the solder and the formation of a solder layer of sufficient thickness and homogeneity takes place uncontrolled. Increasingly higher demands are however now placed on the soldered connection: uniform thickness of the solder layer, distribution of the solder layer over the entire surface of the chip, completely bubblefree, high purity of the soldered connection.
The object of the invention is to further improve the quality of the soldered connection between the semiconductor chip and the substrate.
The method in accordance with the invention for the mounting of semiconductor chips on a substrate having a solder portion is characterized by means of the following steps:
a) Presenting the substrate on a support;
b) Grasping the semiconductor chip by means of a gripper spring mounted on a bondhead;
c) Lowering of the semiconductor chip onto the substrate whereby the gripper carrying the semiconductor chip is deflected towards the bondhead;
d) Lifting the semiconductor chip by a predetermined distance;
e) Releasing the semiconductor chip;
f) Moving the bondhead away.
With step c, first of all the semiconductor chip impacts on the liquid solder whereby the solder portion is at first pressed flat as the result of the impact and then a large part of the solder is pressed out of the gap between the semiconductor chip and the substrate. The solder collects in beads to the side of the semiconductor chip. A gap filled with solder remains between the semiconductor chip and the substrate the thickness of which amounts to only a few micrometres, typically only around 5 micrometres. The aim is that with this step c, the entire back of the semiconductor chip including the comers is wetted with solder and that the solder gathers as uniformly as possible on all four sides of the semiconductor chip. How and whether the wetting of the entire back of the semiconductor chip takes place is not only dependent on the impact speed of the semiconductor chip and the degree of parallelism of its back with the surface of the substrate but also on other factors such as for example the cleanliness of the back of the semiconductor chip and the area on the substrate to be wetted, the form and quality of the solder, etc.
With step d, the semiconductor chip is now, unlike prior art, brought to a predetermined height above the substrate under mechanical control of the gripper whereby solder flows back into the increasing gap between the semiconductor chip and the substrate. The flowing back of the solder takes place under controlled conditions, in particular under the controlled suction effect of the semiconductor chip guided by the gripper. In this way it is achieved that the gap is relatively homogenously filled with solder up to the comers of the semiconductor chip. The wetting is meanwhile concluded, ie, the solder has formed meniscuses and has therefore achieved a stable state. The mechanical connection between the semiconductor chip and the gripper can now be released and the bondhead can be moved away without the position and/or tilt of the semiconductor chip being altered.
It is possible that bubbles, so-called voids, are contained within the solder. By means of a controlled up and down movement of the semiconductor chip after step d, it can if necessary be achieved that possible large bubbles break up into several small bubbles and that the bubbles migrate towards the edge of the semiconductor chip whereby bubbles arriving at the edge disappear. The reduction or even complete elimination of the bubbles, which itself is a characteristic of quality, also has the effect that the position of the semiconductor chip alters less strongly or not at all when releasing from the gripper, ie, improved tilt and therefore a higher value of the thinnest part of the solder layer or a more uniform thickness of the solder layer. As long as larger bubbles exist, there is the danger that the semiconductor chip sinks locally and ends up in a slanted position in relation to the surface of the substrate.
A prerequisite for achieving the required quality of the solder layer is of course that the bondhead is adjusted so that the underneath of the semiconductor chip is parallel to the surface of the substrate.
In the following, embodiments of the invention are explained in more detail based on the drawing.